This invention relates generally to the art of polyolefin films and more particularly to the art of polyolefin film with additives for enhancing slip and block characteristics.
The term polyolefin as it relates to films within this disclosure is meant to include normally solid polymers of the 1-mono-olefins as well as copolymers and ionomers thereof. Films of such polyolefins have a tendency to block upon being wound into rolls. The term "blocking" as used herein refers primarily to the tendency of films or sheets of the polyolefin film to stick or adhere to each other or to other surfaces when such adhesion is otherwise undesirable. When the film is rolled for purposes of storage or shipment, any tendency of the contacting surfaces to block results in breaks, tears and marred surfaces as the rolls are unwound. Similar effects are noted when stacked sheets are pulled apart. The tendency of sheets and rolls of film to block even slightly renders impossible their use in mechanical packaging or wrapping equipment. Blocking results in uneven tension of the film during wrapping, leading to excessive film breakage, unsightly packages, or marred surfaces on the film. The problem of blocking is well recognized by the polyolefin film industry which has provided a standard test therefor by way of ASTM D 1894 which is a technique for determining the coefficient of friction between adjacent film layers.
A technique for reducing the blocking tendency of polyolefin films is the incorporation within the film of an inert particulate filler which has the effect of roughening the surface of the film and thereby significantly lessening the contact area between adjacent layers. Such materials include colloidal silica, diatomaceous earth, clays, aluminum oxide, calcium oxide, magnesium oxide, barium oxide, titanium dioxide, iron oxide, zinc oxide, tin oxide, calcium carbonate, magnesium carbonate, mica, asbestos, calcium silicate, molybdenum disulfide and zinc sulfide.
Another technique for preventing adhesion in wound polyolefin products is the incorporation of slip agents in the film composition prior to film formation. Slip agents are modifiers used in a plastic to provide surface lubrication during and immediately after processing. These agents are added to the plastic during processing and given internal and external lubrication. They have limited compatibility with the film composition and are designed to exude to the film surface and provide an invisible coating on the plastic; thus reducing the coefficient of friction. The primary function of such an additive is to reduce the sticking of the film to itself by minimizing tack.
A major shortcoming, however, of both slip agents and antiblock agents is the adverse effect which each have upon the optical properties and processability of polyolefin film.